The invention relates to a voltage reference source, and in particular, to the voltage reference source operable at a low voltage supply, for example of the order of 1.5V.
For many microelectronics applications, it is extremely desirable to have a voltage reference circuit which would have a simple design and operate at a low voltage supply, for example of the order of 1.5V. Additionally, it would be desired for such a circuit to provide the required voltage temperature dependence and control it independently from the voltage magnitude. Unfortunately, the existing voltage reference circuits hardly satisfy the above requirements which clearly identifies the need for further developments in this area.
An object of this invention is to provide an improved voltage reference source.
According to a first aspect of the invention there is provided a method of forming an output voltage, comprising the steps of:
forming a first voltage V1 which is proportional to a base-emitter voltage Vbe of a transistor (V1=m1Vbe), wherein m1 is a first weight coefficient;
forming a second voltage V2 which is proportional to a thermal voltage VT of a transistor (V2=m2xc2x7VT) wherein m2 is a second weight coefficient;
adding first and second voltages to form the output voltage Vout.
The method further comprises the step of selecting the weight coefficients so as to provide the output voltage having a predetermined magnitude and predetermined function of the temperature. Conveniently, the method comprises the step of selecting the weight coefficients so as to control magnitude and temperature dependence of the output voltage independently. The weight coefficients may be selected so as to provide the output voltage which is independent of the temperature, or alternatively, to provide the output voltage which is an increasing or decreasing function of the temperature. Advantageously, it may be arranged that the increasing or decreasing function of the temperature are linear functions.
To form the first voltage as a fraction of the base-emitter voltage of a transistor, it can be selected that the first weight coefficient is less than unity, i.e. m1 less than 1. For many practical applications it may be selected that the output voltage is proportional to a bandgap voltage, e.g. equal to a fraction of the bandgap voltage, the bandgap voltage being typically 1.244V.
According to a second aspect of the invention, there is provided a method of forming a reference voltage, comprising the steps of:
forming the output voltage Vout as defined above according to the first aspect of the invention; and
adding a base-emitter voltage of a transistor Vbe to the output voltage Vout, thus forming the reference voltage Vr=Vout+Vbe.
Conveniently, it may be arranged that the step of forming the output voltage is performed so as to provide that the output voltage is equal to a fraction of the bandgap voltage.
According to a third aspect of the invention, there is provided a voltage reference source, comprising:
means for forming a first voltage V1 which is proportional to a base-emitter voltage Vbe of a transistor (V1=m1xc2x7Vbe), wherein m1 is a first weight coefficient;
means forming a second voltage V2 which is proportional to a thermal voltage VT of a transistor (V2=m2xc2x7VT), wherein m2 is a second weight coefficient;
means for adding first and second voltages to form the output voltage Vout.
Preferably, the weight coefficients are selected so as to provide the output voltage having a predetermined magnitude and predetermined function of the temperature. Conveniently, it may be arranged that the weight coefficients are selected so that the magnitude and temperature dependence of the output voltage can be controlled independently, e.g. the magnitude of the output voltage is determined by absolute magnitudes of the weight coefficients, and the temperature dependence of the source is controlled by relative magnitudes of the weight coefficients. Depending on circuit requirements, it can be provided that the output voltage is independent of the temperature or has an increasing or decreasing function of temperature, the function being preferably linear functions.
To form the first voltage as a fraction of the base-emitter voltage of a transistor, the first weight coefficient should be less than unity, i.e. m1 less than 1. For many practical applications it may be arranged that the output voltage is proportional to a. bandgap voltage, e.g. equal to a fraction of the bandgap voltage.
Advantageously, the voltage reference source includes three transistors only which are coupled in parallel and balanced with a number of resistors. It allows for a low voltage supply of the circuitry because the lower limit of the voltage supply is defined by only one base-emitter voltage which is typically below 1V.
According to a fourth aspect of the invention there is provided a reference voltage source, comprising:
means for forming the output voltage Vout as defined in accordance with the third aspect of the invention described above; and
means for adding a base-emitter voltage of a transistor Vbe to the output voltage Vout to form the reference voltage Vr=Vout+Vbe.
According to a fifth aspect of the invention there is provided a voltage reference source, comprising:
first, second and third transistors and first to fifth resistors;
collector and base of the first transistor being connected to the base of the second transistor and to a through the first resistor to an output terminal;
collector of the second transistor being connected to the base of the third transistor and through the second resistor to the output terminal;
emitters of the first and third transistors being connected to a negative voltage terminal directly with the emitter of the third transistor being connected to the negative voltage terminal through the third resistor;
collector of the third transistor being connected to the output voltage terminal; and
fourth and fifth resistors being connected across base-emitter junctions of the third and first transistors respectively.
Additionally, the output terminal is connected to a positive voltage terminal through a resistance means or through a current source.
Advantageously, the reference voltage source is operable at a low voltage supply, wherein the low voltage supply is of the order of 1.5V and lower. The voltage reference source circuit includes three transistors only connected in parallel and a number of resistors.